A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements which transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display.
Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. The resolution of a scan line is a result of the directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
In medical ultrasound imaging systems of the type described hereinabove, it is desirable to optimize the SNR. Additional SNR can be used to obtain increased penetration at a given imaging frequency or to improve resolution by facilitating ultrasonic imaging at a higher frequency.
The use of Golay codes in ultrasound is well known in the area of non-destructive evaluation (NDE) using single-element fixed-focus transducers to inspect inanimate objects. Golay codes are also known in the medical ultrasound imaging community. However, the use of Golay codes in an ultrasound imaging system has been dismissed because dynamic focusing, tissue motion (effects not present in NDE) and nonlinear propagation effects were thought to cause unacceptable code degradation with corresponding range degradation.
U.S. Pat. No. 5,984,869 discloses a method and an apparatus for improving the SNR in medical ultrasound imaging by using Golay-encoded excitation of the transducer array. The SNR is improved by transmitting a pair of Golay-encoded base sequences consecutively on each beam at the same focal position and then decoding the beamsummed data. A pair of Golay-encoded base sequences is formed by convolving a base sequence with a Golay code pair after oversampling. A Golay code pair is a pair of binary (+1, -1) sequences with the property that the sum of the autocorrelations of the two sequences is a Kronecker delta function. An oversampled Golay sequence is the Golay sequence with zeroes between each +1 and -1, the number of zeroes being greater than or equal to one less than the length of the base sequence. The aforementioned property of Golay code pairs translates into two important advantages over codes in general: (1) Golay codes have no range sidelobes, and (2) Golay codes can be transmitted using only a bipolar pulser versus a more expensive digital-to-analog converter.
U.S. Pat. No. 5,984,869 further discloses that tissue motion occurring between the transmission of the two sequences of the Golay pair causes code distortion which increases the range sidelobes. By transmitting the second sequence as soon as the echoes from the first sequence are completely received, the time interval between the two transmits can be minimized. Minimization of the interval between transmits in turn minimizes the motion-induced code distortion.
In the aforementioned medical ultrasound B-mode imaging system using Golay-encoded excitation, the frame rate is reduced by a factor of two compared to conventional imaging because two round-trip delayed firings (i.e., two firings with round-trip propagation delay between them) are necessary for each transmit focal zone compared to only one firing per focal zone in conventional imaging. In other words, two focal zones could be beamformed in conventional imaging in the same amount of time it takes to beamform one focal zone using Golay-encoded excitation. The problem to be solved is how to recover the 2.times. frame rate reduction in Golay-encoded excitation (or N.times. reduction in polyphase N-complementary sets in general).
In a paper by B. B. Lee and E. S. Furgason, "Golay Codes for Simultaneous Multi-Mode Operation in Phased Arrays," 1982 IEEE Ultrasonics Symposium, pp. 821-825 (1982), an analog ultrasonic imaging system that transmitted and received two beams simultaneously using orthogonal Golay codes is described. The Lee and Furgason system is a rudimentary analog phased array system capable only of sector scans and incapable of multiple transmit focal zones along a beam direction. Thus there is need for an ultrasonic imaging system using Golay-encoded excitation which improves upon the prior art.